1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine or a printer, and in particular, an improvement in an image forming apparatus in a mode of using a contact transfer member whose transfer medium is sandwiched in a transfer area between the same and an image carrier, and a transfer medium guiding device used therein.
2. Description of the Related Art
Conventionally, as an image forming apparatus of this type, in, for example, electrophotography, ones have already been provided, wherein a transfer device is disposed facing an image carrier such as a photosensitive drum and a toner image electrostatically formed on the image carrier onto a transfer medium by this transfer device.
As transfer devices of this type, non-contact type devices such as corotrons also exist, however, from a point of view of suppressing ozone generation, contact-type devices such as transfer rolls which sandwich a transfer medium in a transfer area between the same and an image carrier have been often utilized.
In such a mode of utilizing a contact-type transfer device, it is required that the transfer medium is securely closely arranged on the image carrier in the transfer area between the image carrier and transfer roll to maintain transferability by the transfer roll satisfactorily.
In order to satisfy such a requirement, conventionally, ones have been known wherein, in order to guide a transfer medium to a transfer area between the image carrier and transfer roll, a transfer medium guiding device has been disposed, in a transfer medium transporting path, at an upstream side of the transfer area. For this transfer medium guiding device, ones having paired guide chutes whereby a transfer medium is made to contact the image carrier at an upstream and outside portion of the transfer area, for pressing the transfer medium to the image carrier side by use of resilience of the transfer medium so as to closely arrange the transfer medium on the image carrier in the transfer area have been known (see JP-A-4-355482, JP-A-10-123848, and JP-A-2003-76154, for example.)
However, in the transfer medium guiding devices of this type as described in JP-A-4-355482, JP-A-10-123848, and JP-A-2003-76154, there are concerns that a gap occurs between the image carrier and transfer medium in the transfer area, wherein a technical problem exists such that, if such a gap exists, an electric discharge is produced by a transfer voltage (current), and deletion and toner scattering occur.
Such inconveniences more clearly appear when a high-triboelectric toner is used than when a low-triboelectric toner is used, since a high transfer current or transfer voltage is required.
The invention has been made in order to solve the above technical problems, and provides an image forming apparatus which can secure adhesion between an image carrier and a contact transfer member to satisfactorily maintain transferability by the contact transfer member and a transfer medium guiding device used therein.
The present inventors have analyzed the above-mentioned technical problems and have obtained the following knowledge.
For example, in the transfer medium guiding device described in JP-A-4-355482, since an angle formed between the contact plane and transfer medium in a contact posture at a contact part of the transfer medium into the image carrier is less than 45°, a pressing force for the transfer medium against the image carrier is weak, and adhesion between the image carrier and transfer medium is likely to become insufficient. Moreover, since a gap between the guide chutes and image carrier is large, the transfer medium easily floats up from the image carrier surface particularly when a transfer medium with a strong resilience is used.
In addition, in the transfer medium guiding device described in JP-A-10-123848, a flexible shielding plate to elastically contact one of the paired guide chutes is provided, blots on a transfer medium transporting surface of the guide chute is prevented by this shielding plate, and the contact posture of the transfer medium is regulated in a direction toward the transfer area by the shielding plate. Accordingly, a pressing force for the transfer medium against the image carrier is weak, and adhesion between the image carrier and transfer medium is likely to become insufficient.
Furthermore, the transfer medium guiding device described in JP-A-2003-76154 is for guiding the transfer material which has been wound around the image carrier to a nipping area, wherein it is apparent that an angle of the transfer medium in a contact posture is less than 45°, and for this, a pressing force for the transfer medium against the image carrier is weak, and adhesion between the image carrier and transfer medium is likely to become insufficient.
Based on such results of analysis, the present inventors have obtained knowledge that adhesion between the image carrier and transfer medium is secured by improving a pressing force for the transfer medium against the image carrier, and the transfer medium with a strong resilience is prevented from floating up from the image carrier by adjusting a lead edge position of the guide chute closer to the transfer area.